The present invention relates to a surface emission type semiconductor laser which emits a laser beam perpendicularly to a semiconductor substrate. Particularly, the present invention relates to a surface emission type semiconductor laser capable of electrically scanning light output in one dimension.
A laser beam scanner for scanning a laser beam generally uses a hologram or galvanomirror, but such a device requires a mechanical movement, and is thus accompanied with vibrations and noise. The device also has drawbacks in that it has high power consumption, difficulties in miniaturization because it comprises an assembly of many parts, and low resistance to an impact.
On the other hand, for example, Japanese Unexamined Patent Publication No. 1-152683 discloses that a semiconductor laser array device comprising a plurality of semiconductor lasers arranged in a line, and a lens system disposed in front of the semiconductor laser array device so that its aperture receives the laser beams emitted from all semiconductor lasers is provided for realizing scanning of a laser beam by changing the position of the emission point on the semiconductor laser array and operating a lens of the lens system. This method requires no mechanical movable mechanism, and thus the above-mentioned drawbacks can be solved.
However, an end surface emission type stripe laser array has a limit to the distance of emission points, and thus cannot be said to be practical from the viewpoints of difficulties in manufacturing and resolution.
Therefore, the use of a surface emission laser array as the semiconductor laser array device permits realization of a laser scanner with high resolution. For example, Japanese Unexamined Patent Publication No. 8-97505 discloses that a resonator is formed by using a layered structure perpendicular to a substrate, and a plurality of contact electrodes are provided on one resonator and separated by forming high-resistance regions so that the electrodes are independently driven to change the position of the emission point.
However, the above-described surface emission type semiconductor laser is suitable for emitting a laser beam from the rear side of the substrate, but has the problems described below with respect to emission of a laser beam from the surface side of the substrate.
The electric currents injected from the contact electrodes are mostly converted into light directly below the contact electrodes. Since the contact electrodes do not transmit a laser beam, the light produced directly below the electrodes cannot be emitted from the back, and is thus finally converted into heat. Namely, all currents injected directly below the contact electrodes become reactive currents. This results in an increase in the threshold value, and a reduction in efficiency. In a conventional structure, the reactive current can be decreased only by decreasing the area of the contact electrodes. However, a decrease in the electrode area causes an increase in contact resistance, thereby decreasing output due to heat generation, and deteriorating crystals and reliability.
As described above, the conventional semiconductor laser beam scanner causes large reactive current if a laser beam is emitted from the back side of the substrate, causing the problem of deteriorating characteristics such as the threshold value, efficiency, output, reliability, etc.
On the other hand, if the laser beam is emitted only from the back of the substrate, a practical problem occurs. Such a device should be used in a state wherein it is mounted on a pedestal, and requires a large number of wiring between the device and a driving circuit. Therefore, the device is turned upside down and subjected to flip chip mounting using solder bumps. However, in the case of flip chip mounting, the pitch of the bonding pads must be as large as at least 180 xcexcm because of the use of the solder bumps. This causes an increase in the dimensions of the device, and the problem of directly affecting the number of devices which can be obtained from one substrate, i.e., the cost. This becomes critical with an increase in the number of electrodes for increasing the laser beam scanning width and resolution.
The present invention has been achieved for solving the above-described problems, and an object of the invention is to provide a light source for a semiconductor laser beam scanner which exhibits a high degree of mounting freedom and high resolution in scanning, and is suitable for emitting a laser beam from the surface side.
(1) The present invention provides a surface emission type semiconductor laser comprising a first reflecting mirror, an active layer, a current narrowing layer, a contact layer, a second reflecting mirror, and contact electrodes, which are formed on a semiconductor substrate, so that a laser beam is emitted perpendicularly to the semiconductor substrate;
wherein the current narrowing layer comprises an AlAs layer and an Al oxide layer formed to surround the AlAs layer;
the AlAs layer is formed in a stripe to form a stripe emission region;
the contact layer is formed in a stripe wider than the stripe emission region to completely cover the stripe emission region; and
a plurality of the contact electrodes are formed independently so as not to overlap the stripe emission region.
The principle of the operation of the surface emission type semiconductor laser of the present invention will be described. When a current is passed through one of the divided contact electrodes, the current is led to be concentrated in the emission region by the current narrowing layer. This is described in further detail below. The current narrowing layer comprises the stripe AlAs layer and the Al oxide layer formed to surround the AlAs layer so that the injected current flows horizontally through the contact layer because the Al oxide layer formed below the contact electrodes is made of an insulator, and is led to the emission region through the stripe AlAs layer made of a conductor. The thus-led current is converted into light by the active layer of the emission region, amplified by a Fabry Perot resonator comprising the first and second mirrors to produce oscillation, and emitted as a laser beam perpendicular to the substrate. When the first mirror has higher reflectance than the second mirror, the laser beam is mainly emitted in the surface direction of the substrate. At this time, the laser emission point is formed on the stripe emission region near the contact electrode through which a current is passed. A current is successively passed through the adjacent independent contact electrodes to move the laser emission point on the stripe emission region.
Particularly, the surface emission type semiconductor laser of the present invention comprises the current narrowing layer and thus does not consume a current directly below the contact electrodes. Namely, it is possible to remove the reactive current. It is also possible to increase the area of contact between the contact layer and the contact electrodes, thereby decreasing contact resistance and generation of heat. Therefore, since the contact electrodes do not intercept a laser beam because the electrodes do not overlap the emission region, the structure of the semiconductor laser is very suitable for emitting a laser beam from the surface side.
Since the laser emission surface and the contact electrodes are in the same plane, material for the pedestal is not limited, and thus the degree of mounting freedom is high.
(2) The region of the contact layer, which is formed on the Al oxide layer, is preferably formed in a comb shape, and the plurality of the independent contact electrodes are preferably formed to respectively contact the comb teeth of the contact layer.
As a result, it is possible to suppress current spreading in the stripe emission region in the direction of the stripe length to increase the current density, thereby further decreasing the threshold value and improving efficiency.
The region of the contact layer, which overlaps the Al oxide layer, contacts the contact electrodes and plays a role as a horizontal passage of a current. The horizontal current component includes a component flowing to the emission region, i.e., a component perpendicular to the stripe, and a component parallel to the stripe. Since the horizontal current component parallel to the stripe excessively decreases the current density in the emission region, it is preferable to remove this component.
Therefore, the region of the contact layer, which overlaps the Al oxide layer, is formed in a comb shape so that one contact electrode contacts only one comb tooth, thereby removing the horizontal current component parallel to the stripe. As a result, it is possible to suppress current spreading in the direction of the stripe length to increase the current density, thereby further decreasing the threshold value and improving efficiency.
(3) The contact layer is preferably a second conduction type in the stripe emission region, and thus a first conduction type region and second conduction type region are alternately formed in the region on the Al oxide layer in the direction of the stripe length so that the plurality of the independent contact electrodes respectively contact the second conduction type regions of the contact layer.
As a result, it is possible to suppress current spreading in the stripe emission region in the direction of the stripe length to increase the current density, thereby further decreasing the threshold value and improving efficiency.
The region of the contact layer, which is formed on the Al oxide layer, contacts the contact electrodes and plays a role as a horizontal passage of a current. The horizontal current component includes a component flowing to the emission region, i.e., a component perpendicular to the stripe, and a component parallel to the stripe. Since the horizontal current component parallel to the stripe excessively decreases the current density in the emission region, it is preferable to remove this component.
Therefore, in the region of the contact layer, which is formed above the Al oxide layer, the first and second conduction type regions are alternately formed in the direction of the stripe length so that one of the contact electrodes contacts only one second conduction type region, thereby removing the horizontal current component parallel to the stripe. As a result, it is possible to suppress current spreading in the stripe emission region in the direction of the stripe length to increase the current density, thereby further decreasing the threshold value and improving efficiency.
(4) The present invention comprises:
the step of forming a first reflecting mirror on a semiconductor substrate;
the step of forming multilayered semiconductor layers comprising at least an active layer, an AlAs layer and a contact layer on the first reflecting mirror;
the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, leaving the contact layer in a stripe;
the step of oxidizing the entire exposed AlAs layer and the AlAs layer below the stripe contact layer by heat treatment at a temperature of 280 to 500xc2x0 C. in a nitrogen atmosphere containing water vapor to convert the AlAs layer to an Al oxide layer, leaving the AlAs layer having a predetermined width, to form a stripe AlAs layer surrounded by the Al oxide layer and having a width smaller than the width of the stripe contact layer;
the step of forming a plurality of independent contact electrodes so that the electrodes contact the upper side of the contact layer and do not overlap the stripe emission region; and
the step of forming a second reflecting mirror on the contact layer so as to completely cover at least the stripe emission region.
(5) In the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, a dry etching method using only an inert gas is preferably used.
(6) The present invention comprises:
the step of forming a first reflecting mirror on a semiconductor substrate;
the step of forming multilayered semiconductor layers comprising at least an active layer, an AlAs layer and a contact layer on the first reflecting mirror;
the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, leaving the contact layer in a stripe;
the step of oxidizing the entire exposed AlAs layer and the AlAs layer below the stripe contact layer by heat treatment at a temperature of 280 to 500xc2x0 C. in a nitrogen atmosphere containing water vapor to convert the AlAs layer to an Al oxide layer, leaving the AlAs layer having a predetermined width, to form a stripe AlAs layer surrounded by the Al oxide layer and having a width smaller than the width of the stripe contact layer;
the step of etching the contact layer on the Al oxide layer to form a comb-shaped contact layer;
the step of forming a plurality of independent contact electrodes so that the electrodes do not overlap the stripe emission region, and respectively contact the surfaces of the comb teeth of the contact layer and correspond to the comb teeth of the contact layer; and
the step of forming a second reflecting mirror on the contact layer so as to completely cover at least the stripe AlAs layer.
(7) In the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, a dry etching method using only an inert gas is preferably used.
(8) The present invention comprises:
the step of forming a first reflecting mirror on a semiconductor substrate;
the step of forming multilayered semiconductor layers comprising at least an active layer, an AlAs layer and a contact layer on the first reflecting mirror;
the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, leaving the contact layer in a stripe having at least one comb-shaped side;
the step of oxidizing the entire exposed AlAs layer and the AlAs layer below the stripe contact layer by heat treatment at a temperature of 280 to 500xc2x0 C. in a nitrogen atmosphere containing water vapor to convert the AlAs layer to an Al oxide layer, leaving the AlAs layer having a predetermined width, to form a stripe AlAs layer surrounded by the Al oxide layer and having a width smaller than the width of the stripe contact layer;
the step of forming a plurality of independent contact electrodes so that the electrodes do not overlap the stripe emission region, and respectively contact the surfaces of the comb teeth of the contact layer and correspond to the comb teeth of the contact layer; and
the step of forming a second reflecting mirror on the contact layer so as to completely cover at least the stripe AlAs layer.
(9) In the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, a dry etching method using only an inert gas is preferably used.
(10) The present invention comprises:
the step of forming a first reflecting mirror comprising a first conduction type semiconductor layer on a semiconductor substrate;
the step of forming multilayered semiconductor layers comprising at least an active layer, a second conduction type AlAs layer and a first conduction type contact layer on the first reflecting mirror;
the step of etching the multilayered semiconductor layers until the AlAs layer is exposed, leaving the contact layer in a stripe;
the step of oxidizing the entire exposed AlAs layer and the AlAs layer below the stripe contact layer by heat treatment at a temperature of 280 to 500xc2x0 C. in a nitrogen atmosphere containing water vapor to convert the AlAs layer to an Al oxide layer, leaving the AlAs layer having a predetermined width, to form a stripe AlAs layer surrounded by the Al oxide layer and having a width smaller than the width of the stripe contact layer;
the step of impurity diffusion for partially making the first conduction type contact layer the second conduction type;
the step of forming a plurality of independent contact electrodes so that the electrodes contact the surface of the second conduction type region of the contact layer, which is formed on the Al oxide layer, and do not overlap the stripe emission region; and
the step of forming a second reflecting mirror on the contact layer so as to completely cover at least the stripe emission region.
A dry etching process for a compound semiconductor generally uses a reactive gas such as Cl2 or the like in order to increase the etching rate. However, in the present invention, particularly a gas having low reactivity, such as an inert gas or the like, is preferably used. The reason for this is the following. In the present invention, etching is finished when the AlAs layer is exposed in a wide range. However, since the AlAs layer has high reactivity, the use of a reactive gas causes excessive reaction of the exposed AlAs layer and the gas remaining on or adhering to the substrate surface in a chamber after the completion of etching, thereby causing surface deterioration.